A Swedish military patrol vessel in the the Stockholm Archipelago, Sweden, Saturday Oct. 18 2014. Sweden's military continues to investigate reports of "foreign underwater activity" in the Stockholm archipelago using high-tech equipped naval vessels, aircraft and home guard forces. (AP Photo/TT News Agency/Pontus Lundahl) SWEDEN OUT
(The Associated Press)

A Swedish Navy fast-attack craft cuts through the water in the the Stockholm Archipelago, Sweden, Saturday Oct. 18 2014. Sweden's military continues to investigate reports of "foreign underwater activity" in the Stockholm archipelago using high-tech equipped naval vessels, aircraft and home guard forces. (AP Photo/TT News Agency/Pontus Lundahl) SWEDEN OUT
(The Associated Press)

A member of the military mans a gun on Swedish Navy fast-attack craft in the the Stockholm Archipelago, Sweden, Saturday Oct. 18 2014. Sweden's military continues to investigate reports of "foreign underwater activity" in the Stockholm archipelago using high-tech equipped naval vessels, aircraft and home guard forces. (AP Photo/TT News Agency/Pontus Lundahl) SWEDEN OUT
(The Associated Press)

Chief of operations Jonas Wikstrom says the information "from a credible source" that sparked the operation Friday was still considered worth pursuing the intelligence operation.

Wikstrom declined Saturday to give details about the operation, except saying the purpose was to find out whether "there had been or still is ongoing foreign underwater activity." He also declined to say whether they had been in contact with other countries.

Neutral Sweden's armed forces have ruled out that it could be some sort of natural phenomenon.

The operation was reminiscent of the Cold War, when Sweden's armed forces routinely hunted for Soviet submarines in its waters.